1. Field of the Invention
This invention relates to quasi-optic grid arrays, such as periodic grid arrays, and in particular to systems for adapting a wave-guide assembly to a reflection-mode quasi-optical grid array.
2. Description of Related Art
Broadband communications, radar and other imaging systems require the transmission of radio frequency (“RF”) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many applications at these high frequencies, a technique called “power combining” has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at higher frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.
Several years ago, spatial power combining using “quasi-optics” was proposed as a potential solution to these problems. The theory was that an array of microwave or millimeter-wave solid state sources placed in a resonator could synchronize to the same frequency and phase, and their outputs would combine in free space, minimizing conductor losses. Furthermore, a planar array could be fabricated monolithically and at shorter wavelengths, thereby enabling potentially thousands of devices to be incorporated on a single wafer or chip.
Since then, numerous quasi-optical devices have been developed, including detectors, multipliers, mixers, and phase shifters. These passive devices continue to be the subject of ongoing research. Over the past few years, however, active quasi-optical devices, namely oscillators and amplifiers, have evolved. One benefit of spatial power combining (over other methods) using quasi-optics is that the output power scales linearly with chip area. Thus, the field of active quasi-optics has attracted considerable attention in a short time, and the growth of the field has been explosive.
A quasi-optical array amplifier is a two-dimensional sheet of active devices that accepts a polarized electromagnetic wave as an input and radiates an amplified output wave with a polarization that is orthogonal to the input polarization. Two array amplifier configurations have been previously reported: transmission-mode arrays and reflection-mode arrays. FIG. 1 shows a typical transmission-mode grid amplifier 10, wherein an array of closely-spaced differential pairs of transistors 14 on an active grid 12 having a front and back side and is sandwiched between an input polarizer 18 and an output polarizer 24. An input signal 16 passes through the horizontally polarized input polarizer 18 and creates an input beam incident from the left (onto the front side) that excites RF currents on the horizontally polarized input antennas 20 of the grid 12. These currents drive the inputs of the transistor pair 14 in the differential mode. The output currents are redirected along the grid's vertically polarized antennas 22, producing, out the back (right) side of the array, a vertically polarized output beam 30 via an output polarizer 24.
Numerous grid amplifiers have since been developed and have proven thus far to have great promise for both military and commercial RF applications and particularly for high frequency, broadband systems that require significant output power levels (e.g.>5 watts) in a small, preferably monolithic, package. Moreover, a resonator can be used to provide feedback to couple the active devices to form a high power oscillator.
Grid amplifiers can be characterized as quasi-plane wave input, quasi-plane wave output (free space) devices. Grid oscillators are essentially quasi-plane wave output devices. However, most microwave and millimeter wave systems transport signals through electrical waveguides, which are devices that have internal wave-guiding cavities bounded by wave-confining, and typically electrically conducting, walls. Consequently, an interface between the two environments is needed in most cases. This interface is needed whether the electric field signal is being fed from a waveguide for effective application to the grid array; or the free space output signal of a grid array is to be collected into a waveguide.
Unfortunately, waveguide-enclosed quasi-optical grid arrays based on transmission-mode architectures are less than ideal. Transmission mode arrays are difficult to mount, because the flat grid arrays must be suspended and precisely aligned in the waveguide while allowing the input and output radiation access to both sides of the array. Another problem is that adequate heat dissipation in transmission-mode configurations, a critical design consideration, especially for high-power, high frequency systems, is difficult to achieve because almost all of the surface area of the array, namely the front and back sides, are used for accepting and delivering the input and output radiation, and thus may not be obscured by a heat dissipater or spreader.
In contrast, reflection-mode arrays require that the radiation have access to only one side of the array. The exemplary reflection-mode array shown in FIG. 2 is a grid amplifier 40 that includes an array of closely-spaced differential pairs of transistors 56 on a two-dimensional active grid 50 that is similar to the active grid 12 used in the transmission-mode architecture shown in FIG. 1. The grid has a front side 52 that is exposed to the environment and a back side 54. The back side of the array is mounted on a reflective mirror. In the example shown in FIG. 2 the mirror doubles as a large heat sink, and is thus referred to as mirror/heat sink component 58. Without passing through a polarizing filter, an input beam 60 (i.e. the signal to be amplified) is incident from the right onto the front side of the array. As in the transmission-mode array, the input beam excites RF currents on the horizontally polarized input antennas of the grid and these currents drive the inputs of the transistor pairs in the differential mode. The currents are redirected along the grid's vertically polarized antennas producing, out the back (left) side 54 of the array. However, in the reflection-mode array, the amplified output beam reflects off of the mirror of the mirror/heat sink component 58 and retransmits back through and out front side 52 of the array to free space, as an orthogonally-polarized output beam 62.
As seen, external polarizers are not needed and heat can be drawn away from the grid via nearly 50% of the array surface, since the entire back side area of the grid array is covered by the heat sink/spreader component 58. The reflection-mode architecture is a particularly attractive alternative to transmission-mode architectures because it can result in a more compact structure with the potential for vastly improved heat dissipation properties. More particularly, each unit cell in the array conducts heat directly though the back side substrate to the heat sink, thereby avoiding large temperature rises in the center of the array.
Unfortunately, however, previously reported implementations of reflection-mode grid amplifiers, See e.g., Lecuyer et al., “A 16-Element Reflection Grid Amplifier,” 2000 IEEE MTT-S Int. Microwave Symp. Dig., pp. 809-812, Boston, Mass. June, 2000, have not fully taken advantage of these potential benefits. One reason is that they have not been integrated into any enclosure. Rather, the input and output signals are typically fed from free space with, for example, radiating horn antennas. Moreover, these implementations were physically large, suffered very high input and output losses, and poor heat dissipation.
Thus, there is a definite need for simple, compact and cost effective integrated waveguide assembly that efficiently mounts and encloses a reflection-mode quasi-optical grid array with improved heat dissipation.